Semiconductor diode lasers are formed of multiple layers of semiconductor materials. The typical semiconductor laser diode includes an n type layer, a p type layer, and an undoped active layer between them such that when the diode is forward biased electrons and holes recombine in the active layer with the resulting emission of light. The layers adjacent to the active layer typically have a lower index of refraction than the active layer and form cladding layers that confine the emitted light to the active layer and sometimes adjacent layers. Crystal facets are typically located at opposite edges of the multilayer structure to provide reflection of the emitted light back and forth in a longitudinal direction, generally in the plane of the layers, to provide lasing action and emission of laser light from one of the facets.
To confine the emitted light laterally, positive-index guides or negative-index guides (antiguiding) structures may be employed in a laser diode array. In a positive-index guide, the refractive index is highest in regions where the laser light has field-intensity peaks and is lower in regions of low field intensity, effectively trapping light within the high-index regions, i.e., the laser elements. In negative-index guiding or antiguiding, the refractive index is lowest in regions where the laser light has maximum field intensity, i.e., the array elements, and is highest in regions that contain relatively low field intensity. Consequently, some of the emitted light will pass into the higher refractive index interelement regions and thus will not be confined to the lasing element regions.
An array of laser emitters can typically oscillate in several possible modes. In a fundamental or zero phase shift array mode, the emitters oscillate in phase, and a far field pattern is produced in which most of the energy is concentrated in a single lobe which is ideally diffraction limited. In general, there are many possible array modes for a multiple element array, and many laser arrays operate in two or three array modes simultaneously and produce one or more beams that are typically two or three times wider than the diffraction limit.
The problems associated with the operation of laser arrays at high power with high beam quality are discussed in U.S. Pat. No. 4,985,897, entitled Semiconductor Laser Array Having High Power and High Beam Quality. That patent describes a laser diode structure, which may be implemented in an antiguided structure, operated at or near a resonance condition in which coupling occurs between all elements of the array.
The development of high-power (greater than one watt) coherent diode laser sources has been an area of continued research efforts. Positive index-guided single-element devices have demonstrated up to 0.6 watt (W) continuous wave (CW) coherent power, with reliable operation demonstrated to only 0.2 W, primarily being limited by the relatively small waveguide width of .apprxeq.3 .mu.m. Single-element antiresonant reflecting optical waveguide (ARROW) lasers have also demonstrated single-mode optical power up to .apprxeq.0.5 W, with the added benefit of a drive-independent beam pattern, due to strong lateral optical-mode confinement in devices of 4-6 .mu.m aperture width. See L. J. Mawst, D. Botez, C. Zmudzinski, and C. Tu, "Design optimization of ARROW-type diode lasers," IEEE Photon. Technol. Lett., Vol. 4, pp. 1204-1206, November 1992. In fact, single-mode ARROW devices with aperture width of up to 10 .mu.m are possible, which should allow for reliable powers of .apprxeq.0.5 W from devices with nonabsorbing mirrors.
Research on phase-locked diode laser arrays in an attempt to increase the aperture width and operating power met with little success in controlling the complicated mode structure until the development of resonant-optical-waveguide (ROW) antiguided arrays. Such arrays are described in D. Botez, L. J. Mawst, G. L. Peterson, and T. J. Roth, "Phase-locked arrays of antiguides: Modal content and discrimination," IEEE J. Quantum Electron., Vol 26, pp. 482-495, March 1990. Antiguided arrays have demonstrated near-diffraction-limited CW operation at 1 W from a 120 .mu.m aperture, with up to 0.6 W in the central lobe of the far-field emission pattern, and reliable operation over 3500 hours has been achieved at 0.5 W CW output, thus making ROW arrays the only high-power coherent device type to date that has demonstrated long-term reliability. However, since such devices are based on meeting a (lateral) optical resonance condition, the fabrication tolerances on their structural parameters have been experimentally and theoretically determined to be very tight, especially as the number of elements increases. See D. Botez, A. Napartovich, and C. Zmudzinski, "Phase-locked arrays of antiguides: Analytical theory II," IEEE J Quantum Electron., Vol. 31, pp. 244-253, February 1995.
Higher, single-mode powers (2-3 W CW) have been demonstrated from "broad-area" type tapered master oscillator power amplifiers (MOPAs), which are in commercial production. However, by their very nature, these "broad area" devices are inherently sensitive to carrier- and thermal-induced index variations that degrade the quality of the beam, and cause the phase correction via external lenses to be drive dependent. See D. Mehuys, L. Goldberg, and D. F. Welch, "5.25-W CW near-diffraction-limited tapered-stripe semiconductor optical amplifier," IEEE Photon. Technol. Lett., Vol. 5, pp. 1179-1182, October 1993, In addition, they have a tendency to break into filaments if the linewidth enhancement factor, .alpha., is greater than two, and/or if nonuniformities are present. J. H. Abeles, R. Amantea, R. Rios, and D. J. Channin, "Finite Difference Beam Propagation Method Modeling for High Power Fanned-Out Amplifier Lasers," Conference Digest of the OSA Optical Design for Photonics Conference, Paper No. PD2, Palm Springs, Calif. 1993; L. Goldberg, M. Surette and D. Mehuys, "Filament formation in tapered GaAlAs optical amplifiers," Appl Phys. Lett , Vol 62, pp 2304-2306, May 1993. For these reasons, reliability remains a question for tapered "broad area" devices.